The present invention relates to an anesthetic vaporizer for the supply of an anesthetic agent to a patient.
It is common to administer anesthetic agents supplied as liquids to a patient as a gas, by vaporizing the liquid, and then carrying the vaporized liquid in a carrier gas. The conditions under which the agent is administered in this way depend on a number of factors, including the anesthetic activity of the agent and the quantity of agent which must be administered to the patient. The conditions can vary between extremes of rates of flow of the carrier gas.
A low flow of carrier gas is associated with closed breathing circuit systems where the carrier gas flow rate is determined by the metabolic gas uptake of the patient, which may be as low as 100 ml./min, and where the gas expired by the patient is returned after the carbon dioxide has been removed. The maximum concentration of anesthetic agent vapor which can be carried by the carrier gas is typically around 30% by volume and is determined by the saturated vapor pressure of the agent.
Carrier gas saturated with agent is mixed with the expired gas in the breathing circuit to produce the required concentration of the agent for inhalation. During surgery, it can be desirable to increase the concentration of the agent in the carrier gas supplied to the patient, for example, because of an increase in the uptake of agent by the patient which can cause the concentration of agent supplied to the patient to fall, or to induce a change in the depth of anesthesia. The limitation on the amount of the agent which can be carried by the carrier gas can be overcome by injecting liquid anesthetic agent directly into the breathing circuit, agent vapor being picked up by the carrier gas as it flows over the liquid agent to increase the concentration in the carrier gas as required.
However, it can be difficult to control accurately the variation with time of the concentration of agent which is supplied to the patient since it will depend on the rate at which agent vapor is picked up by the carrier gas. Furthermore, the injection of liquid agent into the breathing circuit can cause a rapid and undesirable increase in the amount of agent supplied to the patient in the event that some of the liquid is received by the patient as liquid, rather that as vapor in the carrier gas.
Medium and high carrier gas flow rates are associated with open and semi-closed breathing circuits where the range of flow rates will be from about 2 to 15 liters/min. The concentration of anesthetic agent in the carrier gas is generally in the range of 0 to 5% by volume and, under these conditions, there is little variation in the agent concentration due to the factors such as an increase in the uptake of agent by the patient.
However, in order to provide a steady output of concentration of agent, it is necessary for large quantities of the agent to be vaporized. This can be achieved by providing a chamber in which the agent is vaporized and mixed with the carrier gas. It is highly preferable that the agent be completely vaporized by the vaporizer since supply of incompletely vaporized agent can lead to accumulation of liquid anesthetic agent in the chamber in which the agent is vaporized and mixed with the carrier gas. This, in turn, can lead to significant fluctuations in the rate at which the agent is administered to a patient; as an agent vaporizes, the effective amount of agent administered to the patient is greater than intended. Furthermore, the collection of liquid agent makes flushing of agent out of the apparatus more difficult.
Unfortunately, however, the use of the aforesaid chamber in which the agent is vaporized and mixed with carrier gas cannot be used when the flow rate of the carrier gas is low, for example less that about 0.8 liters/min., since it exacerbates the problem associated with low flow rate regimes of controlling changes in agent concentrations accurately.
It would obviously be advantageous to have a single machine be used to administer an anesthetic agent to a patient in a carrier gas, whether the flow rate of the carrier gas is low or high, that is, achieve the best of both systems in a single machine, however, the differing requirements as explained, of the two regimes makes this difficult.
In U.S. Pat. No. 4,611,590, there is disclosed an apparatus for adding a liquid anesthetic agent to respiratory gas supplied to a patient, in which the agent is supplied to a chamber in which it is heated to cause vaporization for subsequent supply directly to a patients breathing circuit. Alternatively, the agent can be supplied to a swirl chamber where it is mixed with a carrier gas. From the swirl chamber, the agent with its associated carrier gas is administered directly to the patients circuit. The apparatus therefore provides the possibility either of administering the anesthetic agent carried by a large volume of a carrier gas or directly without any carrier gas. However, the apparatus of that patent requires the use of the combination of a heated chamber, a temperature sensor and a heated conduit to achieve its operating characteristics.